I made a very simple calculator in excel a while back to do some simple calculations like this. In your sister's case, the bulb would pay for itself in two weeks (assuming 10 cents per kWh)! My calculator doesn't include bulb replacement costs, but an incandescent normally lasts 800 hours (a little over 1 month if its never turned off). A CFL lasts on average 10,000 hours (about 14 months if its never turned off). Over a years time, you'd go through about 11 incandescent bulbs and only 1 CFL. So, she would go through $5.50 in incandescent bulbs (if they are around $.50, I dunno, I don't buy them anymore). So, in a year she would save about $45 by replacing ONE bulb. Not bad at all.
Here is my experience with CFLs when I moved into my house. By doing my whole house, they are saving me almost $30 a month and reduced my electric usage by 40%. It cost about $100 to do the whole house. Payback is 3.5 months roughly.
CFLs Lower Energy Usage | EcoRenovator.org
I attached the calculator if you want to play with it. Just fill in the yellowed cells and everything else will populate.
